Particulate matter filters are increasingly used in automotive emissions systems for reducing particulate concentrations in engine exhaust. When soot accumulates to a threshold level on the particulate filter, a filter regeneration process may be used to burn off the accumulated soot under controlled engine operating conditions. However, over time, such particulates filters can suffer irreversible decreases in trapping efficiencies as the filter develops cracks due to uncontrolled temperature excursion during the filter regeneration process. Losses in trapping efficiency of the particulate filter may result in increased particulate matter emissions well above the regulated limit.
Increasingly stringent particulate matter emissions standards and proposed government-mandated on-board diagnostic (OBD) requirements for monitoring the trapping efficiency of a particulate filter have stimulated much research into new techniques for monitoring particulate filter performance. One method includes determining a differential pressure across a particulate filter. If the differential pressure is less than a threshold differential pressure, then the particulate filter may be leaking. However, this method may not be suitable for detecting a failure of the filter due to interference effects from ash loading in the filter. Other methods to determine particulate filter leakage include utilizing a soot sensor, located downstream of a particulate filter, to monitor a soot load in exhaust flow and signaling when the soot load exceeds a soot threshold (e.g., the soot threshold may be based on a threshold amount of acceptable soot leakage based on particulate matter emissions).
However, the inventors herein have recognized potential issues with such systems. As one example, the soot sensor may have low sensitivity to leaked soot due to a relatively small portion of soot being deposited on the soot sensor. This may be due to an exhaust pipe geometry and/or poor mixing of the exhaust gas. Furthermore, large diesel particulates and/or water droplets may impinge onto surfaces of the soot sensor, altering the soot sensor reading.
In one example, the issues described above may be addressed by a system for a particulate matter sensor disposed along an exhaust passage comprising an outer tube coaxial with an inner tube having a concave bottom with a central opening, and where perforations of the inner tube face a curved sensor element located in an annular space between the outer tube and the inner tube. In this way, exhaust gas may flow through the perforations to the sensor element to accurately determine a condition of a particulate filter upstream of the particulate matter sensor in the exhaust passage.
As one example, the perforations face a first surface of the sensor element comprising a pair of interdigitated electrodes spaced apart from one another. There may be a heating element coupled to a second surface of the sensor substrate facing an interior wall of the outer tube. The sensor substrate is configured to capture soot where the soot may be deposited between the pair of interdigitated electrodes. As the soot accumulates, the separated electrodes may become bridged (e.g., electrically coupled), decreasing an electrical resistance of one of the two electrodes. In response to the electrical resistance being decreased, the heating element may be activated to burn off accumulated soot on the sensor substrate. Additionally, a particulate filter in the exhaust passage upstream of the particulate matter sensor may be regenerated in response to the electrodes becoming bridged. A time interval between subsequent regenerations of the sensor substrate may be measured, where a degradation of the particulate filter may be determined based on the time interval being less than a threshold time interval.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.